This invention relates to treatment of spent water supply. In particular it relates to treating primary sewage effluent in an intermediate system to effect biological removal of oxidizible sewage solids. The typical community sewage plant for treatment of residential and/or industrial waste water includes three categories of treatment system, usually designated primary, secondary and tertiary.
The primary treatment consists of any operations, such as screening or sedimentation, that remove particles above colloidal size. It also usually removes some 30-60% of the BOD. Removal of colloidal or dissolved materials, and further reduction of the BOD, is accomplished by secondary treatment, which is biological, by encouraging the growth of microorganisms that utilize waste material in the sewage as food. One commonly used definition of tertiary treatment is any treatment in addition to secondary treatment, such as disinfection.
In the primary treatment stage the sewage is moved by gravity flow or by pumping. Flow velocities in the pipe are usually maintained above a minimum of 70 cm/sec. in order that solids do not settle out in the pipe. Low flow velocities, long detention times, and relatively high temperatures have caused treatment difficulties. It is customary to have a means of bypassing the plant during periods of flow that exceed the hydraulic capacity of the plant. Generally multiple units are provided for each stage of treatment. Thus, during periods of routine maintenance or repairs, it is not necessary to by-pass this treatment stage. Protection is given to pumps against large objects in sewage by employing coarse racks with clear openings greater than 5 cm. Mechanically cleaned racks allow smaller clear openings because hydraulic head loss is low. Mechanical cleaning can be either continuous or intermittent. Comminutors may be used to macerate floating material into sizes sufficiently small (for example, less than 1 cm) so that particles will not clog centrifugal pumps.
Grit chambers are sometimes placed ahead of sedimentation chambers to remove heavy solids without removing finer sediment, to prevent excessive wear in pumps, and for protection against loss of volumetric capacity.
In the sedimentation tanks (settling tanks) the smaller solids settle, and oil and grease, which are lighter than water, float and can be skimmed and taken to the sludge digester.
There are two main processes utilized for biological secondary treatment of waste water. They are the trickling filter, and the activated sludge process.
Present-day biological treatment methods are basic processes evolved over the years, but the underlying principles remain unchanged. The basis is the formation of a suitable environment so that microorganisms may thrive under controlled conditions. The microorganisms may come from the sewage itself. The suitable environment is one which is rich in food and maintained in an aerobic state.
Intermittent sand filters are much like slow and filters used in potable water treatment. The sewage is applied to the sandy area and allowed to flow slowly downward. Surface accumulations of solids are periodically removed. Biological films that form on the sand grains undergo continuous stabilization and it is usually necessary to rest the beds between dosings so that objectionable conditions do not develop.
The trickling filter is a construction of stones (or other coarse material) over which the sewage flows. This process is probably the most widely used aerobic biological treatment method. Sewage is distributed by slowly rotating arms equipped with nozzles and deflectors, and allowed to flow slowly over the filter stones. Air is drawn into the filter by temperature differential, thus keeping a supply of oxygen.
The filter medium may be stone or plastic filter media (2-10 cm). These stones permit sufficiently loose packing to allow free flow of water and air with sufficient openings to prevent clogging by biological slimes. Sewage flows slowly downward over the filter medium and the effluent is collected in tile underdrains, which provide for collection of filter effluent and circulation of air into the filter. The underdrains discharge into a main collection channel which in turn discharges to a final settling tank before tertiary treatment.
In contrast to the trickling-filter process, the activated sludge floc is suspended in the moving stream. This process originated in attempts to purify sewage by blowing air into it. It was observed that after prolonged aeration of sewage in a tank, flocs composed of voraciously feeding organisms developed. When aeration was stopped, this floc settled. Addition of fresh sewage to the tank containing the sludge produced high purification in a reasonable time. Thus, the name given the floc was activated sludge. As was the case with the trickling filter, the activated sludge process may be operated as a fill-and-draw system or continuous operation can be employed. The process involves the return of some of the activated sludge to the aeration tank influent and discharge of excess sludge to digestion, aeration of the sludge-sewage mixture to attain purification, and settling of the aeration tank effluent to remove floc from the plant effluent. Floc is formed in sewage by aerobic growth of unicellular and filamentous bacteria. Protozoa, bacteria and other aerobic or facultive anaerobic organisms are found in the floc matrix. These organisms gain food and energy by feeding upon the sewage. In this aerobic process, air requirements are high because oxygen is only slightly soluble in water (10 mg/liter).
The activated sludge process of treating sewage is well known, as are modifications, such as the "contact stabilization" process. These processes employ aerobic biological stabilization of sewage pollutants. The activated sludge of these processes is a flocculent, heterogenous mixture of inert materials and microorganisms. The processes by which biodegradation take place usually employ aerobic micro-organisms, those species which require oxygen for living and growing. Bio-oxidation of sewage can also be effected in the presence of facultive anaerobes, which are microorganisms normally using free oxygen, but which can live with little or none.
The micro-organisms of primary significance are bacteria and protozoa. When untreated sewage is mixed with activated sludge, the micro-organisms in the sludge stabilize the biodegradable organic materials of the sewage by metabolism, producing carbon dioxide, water, and newly synthesized microbial cells or activated sludge. Separation of the activated sludge from the water by sedimentation produces a clear, supernatant liquid that can be safely discharged to a receiving stream or river or the like, with or without tertiary treatment.
The activated sludge which settles out by sedimentation is normally retained in the system for mixing with additional untreated sewage. After the plant or system has been in operation for a period of time, however, it becomes necessary to dispose of some of the accumulated sludge. The sludge mess can be significantly reduced by aerating it for an extended period of time in the absence of organic food or sewage by a process known as endogenous respiration or aerobic digestation. It is similar to basal metabolism in animals; that is, the microbes literally eat or burn themselves up. Ultimate disposal of the digested sludge ash can be safely carried out by spreading it on agricultural land, for example. A complete description of secondary sewage treatment is given in U.S. Pat. Nos. 3,355,023; 3,654,146; 3,769,204; 3,803,029; 3,794,581; 3,804,255; 3,812,032; 3,817,857 and 3,812,512, incorporated herein by reference.
Biological oxidation methods have mostly employed air as the large oxygen source. The quantity of air required to supply oxygen is primarily due to the 4/1 dilution with nitrogen, and typically only 5-10% of the oxygen is absorbed due to the low oxygen mass transfer efficiency of the method. The large amount of energy supplied to the air is normally sufficient to mix and suspend the bacterial solids in the liquid.
The direct use of oxygen instead of air in treatment of municipal and chemical wastes has been developed because of its potential advantages in reducing the quantity of required gas, primarily due to retention time reduction. Because of the additional cost of oxygen, it must be used sparingly and effectively. This necessitates a small volumetric ratio of gas-to-liquid as compared to air aeration. The partial pressure of oxygen in the aerating gas must be sustained at high level to achieve economics in the cost and operation of aeration equipment while still obtaining high rates of oxygen dissolution. Although prior art systems can be designed to achieve a high percentage oxygen absorption, they are not readily adapted to the handling of mixed liquid-solid suspensions such as encountered in the activated sludge process for waste water treatment. Neither are the conventional systems suited for contacting large volumes of liquid and small volumes of gas with high rates of dissolution and with low energy consumption.
The achievement of both high oxygen utilization and high oxygen partial pressure in biological oxidation is further complicated by the evolution of diluent gases from the mixed liquor undergoing aeration. Usually the BOD-containing feed water to the process is nitrogen-saturated with respect to air. While mass transfer of nitrogen is not a consideration when air aeration is employed, it becomes a very significant factor when the nitrogen content of the aeration gas is reduced and the volume of aeration gas becomes small. This is because the dissolved nitrogen will be stripped from the liquid into the gas and will reduce the oxygen partial pressure of the gas. Other gases evolved from the liquid which are inert to the biochemical reaction will have a similar effect, e.g., argon and moisture. Carbon dioxide, which is a product of the oxidation, will also evolve in substantial quantity and further suppress the oxygen partial pressure. The use of oxygen-enriched secondary sewage treatment is disclosed in U.S. Pat. Nos. 3,547,812; 3,547,814; 3,547,815 and 3,660,277, incorporated herein by reference. If an oxygen-enriched aeration gas is utilized effectively, then its volume relative to air will be very low, e.g., 1/90. While this offers opportunities for cost savings in gas compression, it presents problems in liquid mixing and of oxygen dilution with impurities. The total energy input to the small quantity of gas for purposes of oxygen solution may now be far less than that required for suspending and mixing the solids in the liquid. The inert gases evolved from the liquid will also impair the oxygen partial pressure to a greater extent as the quantity of aeration gas is reduced.